The present invention relates to a mobile communication system such as an automobile telephone system or portable telephone system and, more particularly, to a mobile communication system characterized by channel assignment based on cellular channel assignment.
In a mobile communication system such as an automobile telephone system or portable telephone system, the service area is generally covered by a plurality of base stations. In this case, in assigning frequencies to base stations which cause no interference, the same frequency as that used in one base station is repeatedly assigned to the other base station, thereby realizing efficient use of frequencies. Such a scheme is called a cellular scheme.
Channel assignment schemes which are used in each base station when the cellular scheme is employed are roughly categorized into two schemes. The first scheme is called a fixed channel assignment scheme. In this scheme, on the basis of the estimation result of the propagation characteristics of signals, channels to be used in each base station are permanently fixed thereto in advance so as to prevent interference. The fixed channel assignment scheme is generally used in the current automobile telephone or portable telephone system.
The second scheme is called a dynamic channel assignment scheme. In the dynamic channel assignment scheme, a channel on which no interference occurs is selected and used for each communication. With the use of the dynamic channel assignment method, although the method and apparatus arrangement for channel assignment are complicated, all channels can be used as long as no interference occurs. The dynamic channel assignment scheme is therefore larger in traffic handling capacity than the fixed channel assignment scheme. Owing to such an advantage, attempts have been made to use the dynamic channel assignment scheme in an automobile telephone or portable telephone system and an outdoor cordless telephone system which will be commercially available in the future.
The repetition distance of a radio channel in the fixed channel assignment scheme is determined on the basis of the carrier to interference (C/I) ratio at the boundary between cells. Since the power of a desired carrier is high near the center of the cell, a radio channel can be used at a shorter repetition distance than near the boundary between the cells.
The following technique has therefore been proposed. The cell is divided into small annular cells in accordance with the distance from the base station. The reuse distance of a channel assigned to a small cell near the base station is set to be shorter than that of a channel assigned to a small cell remote from the base station, thereby increasing the frequency utilization efficiency. This technique is called reuse partitioning and disclosed in William C. Y. Lee, "Mobile Cellular Telecommunications Systems", McGraw-Hill Book, pp. 259-261, 1989.
With regards to the dynamic channel assignment scheme, various algorithms for selecting communication channels have been proposed. Of these proposals, a scheme called ARP (Autonomous Reuse Partitioning) is known, which can dynamically realize reuse partitioning without performing complicated control, and allows high frequency utilization efficiency. ARP is described in Japanese Patent Laid-Open No. 4-351126 and Toshihiro Kanai, "Autonomous Reuse Partitioning in Cellular Systems", Conference Record of 42nd IEEE VTC, February 1992, pp. 782-785.
In this ARP scheme, when a speech communication request (to be referred to as a call hereinafter) is transmitted from or to a mobile station in a cell, a base station belonging to the cell measures the C/I ratios of each idle channel at both the base station and the mobile station in the order commonly set for all the cells. The base station then assigns the first channel on which this C/I ratio exceeds the first level as a predetermined value for communication. With the use of this technique, the distances between base stations and mobile stations using identical channels are made almost equal, thus dynamically performing reuse partitioning. With this operation, the reuse distances of identical channels can be decreased to increase the frequency utilization efficiency.
As a dynamic channel assignment scheme, a channel segregation scheme has also been proposed. In this scheme, each base station assigns a priority to each channel, and increases/decreases the priority of each channel on the basis of the measurement result of the I/C ratio thereof. The channels are selected in the order of decreasing priorities. The channel segregation scheme is disclosed in Yukitsuna Furuya and Yoshihiko Akaiwa, "Channel Segregation, a Distributed Adaptive Channel Allocation Scheme for Mobile Communication Systems", IEICE Transactions, vol. E 74, No. 6, June 1991, pp. 1531-1537. According to the scheme, in a cell, each base station learns about a channel resistant to interference from the past utilized states of channels. Since the channel resistant to interference is preferentially used, the occurrence probability of interference (interference probability) can be decreased.
According to the ARP scheme as a dynamic channel assignment scheme, as described above, reuse partitioning is dynamically performed to decrease the reuse distances of identical channels, thereby increasing the frequency utilization efficiency. However, the C/I ratio of a channel assigned by the ARP scheme exhibits a small margin with respect to the first level. For this reason, the number of times of channel handoff (to be referred to as the number of times of interference) during speech communication increases owing to the influences of interference, and the number of times forced termination of speech communication (to be referred to as forced disconnection) increases owing to channel handoff failures.
According to the channel segregation scheme, the priority of a channel frequently used in a given base station increases by learning, but the priority of the identical channel in an adjoining base station decreases. As a result, the frequency of using the identical channel in the adjoining base station decreases to decrease the number of times of interference. However, since the reuse distance of the identical channel increases, the frequency utilization efficiency decreases.